1. Field of the Invention
This invention relates to the preparation of rigid cellular polymer materials, particularly polyisocyanurate and polyurethane foams, characterized by improved insulating properties and a reduced blowing agent requirement. More particularly, the invention relates to the preparation of these improved foams by using a polyisocyanate prepolymer derived from a polyester polyol made from ethylene glycol.
2. Description of the Prior Art
The preparation of foams characterized by isocyanurate and urethane linkages is well known in the art. Generally, these foams are prepared by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a catalyst(s). Polyester polyols of many types can be used as the polyol components in the production of these foams.
Because of the relatively high cost of various typical polyols useful in making cellular polymers and because an inexpensive source of polyols is now available from such sources as crude dimethyl terephthalate oxidate residues, recycled poly (ethylene terephthalates), and various other scrap polyester sources, efforts have heretofore been directed to processes for converting these materials to commercially useful polyester polyols. These efforts have led to a number of polyester polyols found to be highly suitable in producing foams of good quality.
It is also known that polyols, including certain aromatic polyester polyols, can be reacted with isocyanates to form isocyanate-terminated quasi-prepolymers which in turn can be used in the formation of polyurethane and/or polyisocyanurate foams, as disclosed, e.g., in U.S. Pat. No. 4,791,148.
A most important characteristic of rigid foams is their insulating value which is expressed by the k- and aged k- factor. The k-factor is a measurement of the insulating properties (thermoconductivity) and the aged k-factor indicates insulating properties over time. The foams of U.S. Pat. No. 4,791,148 which are prepared from polyisocyanate prepolymers derived from diethylene glycol-based polyester polyols exhibit low initial thermal conductivity values and maintain low thermal conductivity values after prolonged periods of use.
The insulating value of the rigid foams is dependent, among other things, on the type and amount of the blowing agent(s) employed in their preparation. Since these agents make up a considerable proportion of the foam and can add significantly to its cost, any way of bringing about a reduction in their amount needed in the foam production while still yielding the above-noted low initial and aged thermal conductivity values would be very advantageous. Additionally, the chlorofluorocarbon blowing agents have come under attack in recent years on the grounds that they give rise to environmental problems concerned with ozone depletion in the stratosphere, and efforts are ongoing to find ways of reducing their use in foam production.